Graphite oxide is a class of an intercalated layered material that is prepared by oxidizing a graphite with an oxidizing agent such as, for example, a chlorate salt or a permanganate salt. Graphite oxides are especially useful as intermediates in the preparation of exfoliated graphites, including graphene. One method of preparing an exfoliated graphite is rapidly heating a graphite oxide to high temperatures as exemplified later herein.
Exfoliated graphite, also known as expanded graphite, generally is an exfoliated or partially delaminated graphite having a Brunauer-Emmett-Teller (BET) surface area greater than BET surface area of graphite but less than the BET theoretical surface area of a single graphene sheet (i.e., less than 2630 square meters per gram (m2/g)). Conventional exfoliated graphite typically has a BET surface area of between about 15 m2/g to about 150 m2/g, depending on average particle size. Exfoliated graphite is useful for a variety of applications including, for example, in the formation of composites such as polymer composites and the higher the BET surface area of the exfoliated graphite, the more useful it is in such composites.
One disadvantage of long-known chlorate salt-based graphite oxidation reaction processes is nitric acid in reaction mixtures. A portion of the nitric acid evaporates during the reaction. The evaporated nitric acid then condenses in gas outlet lines. The process also generates hazardous chlorine dioxide (ClO2) gas, which is believed to be associated with effective oxidation of the graphite to graphite oxide. But during the reaction, concentration of the ClO2 gas often builds up from headspace of reaction vessels, and nitric acid condensate in gas outlet lines has previously inhibited adequate venting of the ClO2 gas from the headspace, leading the concentration of the chlorine dioxide to more quickly increase to explosive concentrations (i.e., 10% or higher, for example, 50%) in headspace of the reaction vessel. Further, the evaporated nitric acid corrodes reaction vessel equipment such as, for example, a cell or window in the reaction vessel that allows monitoring of progress of the reaction in the reaction mixture, or concentration of ClO2 gas in the headspace of reaction vessel, by spectroscopic means (e.g., ultraviolet (UV) spectroscopy). To minimize such hazards and the disadvantage of using nitric acid, temperature of the reaction mixture is typically kept at about 0 degrees Celsius (° C.) to 5° C., but this temperature reduces solubility and reactivity of the chlorate salt and can also increase concentration of the chlorine dioxide in the reaction vessel when the reaction mixtures are warmed or allowed to warm to a higher temperature (typically ambient, i.e., about 25° C.) after addition of the chlorate salt. There is a need in the art for a graphite oxidation process that eliminates use of nitric acid without reducing quality of graphite oxide produced thereby.